Modeling of bubble flows in vertical pipes with the N-phase compressible algebraic slip model

Abstract

In the current work, the N-phase compressible algebraic slip model (ASM) framework is presented. The framework is implemented in TransAT© which is a commercial finite-volume CFD solver specialized in the modeling of multiphase flows. It is applied to bubbly flows in vertical pipes where compressibility of the gas phase is important. Effects of compressibility of the gas phase namely, the change in volume of the individual bubbles, the reduction in the liquid volume fraction, and the change in bubble size due to almost constant number density are identified and illustrated with the help of an analytical one-dimensional model. It was shown that the volume effect was the strongest, although the effect of the change in the bubble radius is also important though to a lesser extent.The framework implemented in TransAT© does not solve for the gas number density and assumes a fixed radius for the bubbles. In the first step the implementation is verified with the help of the one-dimensional model. In the next step, the implementation is tested against the selected test cases from the TOPFLOW database for different types of bubbly flow namely the wall-peaked, the transition and the central-peaked. The simulation results are able to predict the overall behavior of the different types of bubbly flow. The radial gas velocity profiles for all the test cases showed a good match with the experimental data. Unrealistic accumulation of small bubbles close to the walls is observed for the wall-peaked test cases. The results showed that the unrealistic behaviour could be removed tuning the wall lubrication force model. The results highlight the inadequacy of the current wall lubrication force models and show that the wall lubrication force should depend on the velocity of the continuous phase probably in an analogous way to the lift force.